1. Field of the Invention
The present invention relates generally to information storage using holographic media, and particularly to parallel recording and replication of holographically stored data.
2. Background of the Invention
Traditionally, high capacity data storage drives/media (e.g., magnetic tape, magnetic disk, optical disk and the like) has presented a scale of growth in data capacity that has outpaced the scale of growth in data transfer rates. Data transfer rate limitations primarily arise from the reading and writing of data in a substantially serial fashion. As a result, the time it takes to access all the data in a medium is exponentially increasing (e.g., in a StorageTek™ 9940 tape, with a capacity of 200 GB, and a data transfer rate of 30 MB/s, the resulting total access time is 1.85 hours).
This presents a significant problem for applications that require substantial copying of the contents of media, such as in the cases of archival duplication (safely storing an archive at an alternate location); system state snapshots (allowing the recovery of a computing system to a previous state); system mirroring (for quick disaster recovery); data distribution (allowing the distribution of a limited number of copies of the media); archive restoration (mass copying of an archive when the media approaches the end of its lifetime), among others.
One implementation of holographic data storage (HDS) provides a data handling advantage by storing data in the form of bit arrays termed “pages.” These data pages typically consist of thousands to millions of data bits which are written and/or read in a single step. Furthermore, HDS provides an advantage in storage density by multiplexing several data pages in the same volume of the medium. The optical multiplexing, fortuitously, also allows parallel access to multiple data pages stored in a given medium. Thus, the data can be massively parallel accessed either by illuminating a single location using all reference beams, or by illuminating the entire medium with a given reference beam at a time, the exact addressing method depending on the multiplexing scheme used during recording.
If data could be copied within this highly parallel optical domain, the data rate limitations of today's serial data handling storage devices would be overcome. However, such parallel copy processes would have limitations.
Similar to the process of photocopying paper documents, the copies (and copies of the copies) present a lower signal-to-noise ratio (SNR) than the original. Today's data storage systems provide electronic signal processing (typically by digital processes after an analog to digital conversion of the data has occurred) to maintain the integrity of the data by preserving its SNR. One alternative would be to apply this same type of electronic detection to every HDS bit before recording it optically. This method, however, suffers from the electronic transfer rate limitation described above. What is needed is a system and method to provide signal processing in the optical domain that maintains the HDS data rate performance advantage while also maintaining the SNR of the copied data.